Communication systems, especially of a non-wired type, share a common faith in that they have to cope with the available bandwidth over the transmission path used. Numerous techniques have been developed through the years to utilize the available bandwidth as efficient as possible in order to enhance the bit rate over the transmission path. One of these techniques is so called space division multiplexing (SDM) by which a communication signal is fed and divided over a number of separate transmission paths in parallel. The communication means of the type described in the opening paragraph employ this technique and for that purpose are equipped with transmission and reception means for every transmission path which is used for the exchange of the communication signal.
A problem encountered with SDM in a non-wired environment is that the signals sent by the different transmitters are likely to interfere with each other such that each receiver not only receives a signal from the associated transmitter but also from the other transmitters. In a simple case of two transmitters and two receivers the signals r1 and r2 received by the two receivers may be represented as follows:
            r      1        =                            h          1                ·                  x          1                    +                        h          2                ·                  x          2                                r      2        =                            h          3                ·                  x          1                    +                        h          4                ·                  x          2                    where x1 and x2 are the signals sent by both transmitters and h1.4 are still unknown constants representing distortion and attenuation factors as well as other environmental and atmospheric influences exerted on the transmitted signals during transport. More generally this may be represented as: ri=Hi.xi, where Hi is the appropriate column of a n-dimensional matrix containing the constants hj, said dimension n being equal to the number of transmission paths used. If these constants are known, the system will be able to satisfactorily decode the signals received by the receivers and to derive the original signals xi out of them. Accordingly it is necessary to teach the system these factors before information is being sent. To this end the system is provided with training means which are capable of issuing known training codes which are packed into one or more training symbols which precede the information to be sent. Based on these training codes, the reception means will be able to calculate the constants hj and to apply these constants to the information which follows.
A straightforward training scheme would be to use a predetermined training symbol and to send at least one such symbol by each transmitter consecutively while the other transmitter(s) are inactive. In this manner the receiver may calculate the first column of the above matrix from the training symbol sent by the first transmitter, the second column from the training symbol sent by the second transmitter, and so on. Hence, a system using n transmission paths will require a minimum of n training symbols to recover the constants hj. This training length is a serious problem for high rate wireless packet transmission links, because of the associated overhead which reduces the net data rate. For instance, at 100 Mbps, a 1000 byte packet has a duration of 80 μs. Using the same 4 μs symbol duration as the IEEE 802.11a standard, a system using 4 transmission paths would take a minimum training time of 16 μs, which means a significant overhead of 20%. The training overhead grows with bit rate and number of transmission paths so it partially reduces the benefits of using SDM.
The present invention has inter alia for its object to provide a communication system which the training time may be reduced considerably compared to the above. It is a further object of the invention to provide such a communication system with improved frame detection and frequency synchronization.